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The U.S. Food and Drug Administration called on drug companies to help limit the use of antibiotics in farm animals, a decades-old practice that scientists say has contributed to a surge in dangerous, drug-resistant bacteria.

Antibiotic drugs like penicillin are routinely mixed with animal feed and water to help livestock, pigs and chickens put on weight and stay healthy in crowded barns. Scientists have warned that such use leads to the growth of antibiotic-resistant germs that can be passed on to humans.

The FDA has struggled for decades with how to tackle the problem because the agriculture industry argues the drugs are a key part of modern meat production.

Under the new FDA guidelines, the agency recommends antibiotics be used "judiciously," or only when necessary to keep animals healthy. The agency also wants to require a veterinarian to prescribe the drugs. They can currently be purchased over-the-counter by farmers.

"Now you have a veterinarian who will be consulting and providing advice to these producers, and we feel that is an important element to assure that they are in fact using these drugs appropriately," said William Flynn, a deputy director in FDA's veterinary medicine centre.

Veterinary prescriptions

Last year, members of the Canadian Medical Association voted to support mandatory veterinary prescriptions for antibiotics in animals. Representatives for the cattle and poultry industries told the parliamentary health committee that producers use antibiotics carefully because they're expensive, and that most are prescribed.

When CBC's Marketplace tested 100 samples of retail chicken from across Canada, a significant number of antibiotic-resistant bacteria like E. coli and salmonella were found, including bugs resistant to front-line antibiotics used in humans.

The draft recommendations by the FDA are not binding, and the agency is asking drug manufacturers' to voluntarily put the proposed limits in place.

Drug companies would need to adjust the labeling of their antibiotics to remove so-called production uses of the drugs. Production uses include increased weight gain and accelerated growth, which helps farmers save money by reducing feed costs.

The FDA hopes drugmakers will phase out language promoting non-medical uses within three years.

"This is the most sweeping action the agency has undertaken in this area, as this covers all antibiotics used in meat and poultry production that are important to human health," said Laura Rogers, director of the Pew Charitable Trusts' campaign on industrial farming.

Voluntary limits

But some public health advocates said they do not trust the drug industry to voluntarily restrict its own products. FDA officials said that a formal ban would have required individual hearings for each drug, which could take decades.

"The process we would have to go through is a formal hearing process, product-by-product that is extremely cumbersome," said Mike Taylor, FDA Commissioner for foods. "There's no point in going through those legalistic proceedings when companies are willing to make this shift voluntarily."

Taylor said the FDA has consulted closely with drugmakers, and expects them to support the measures.

An estimated 80 per cent of all antibiotics sold in the U.S. wind up on animal farms. Neither industry nor the government tracks what percentage of those drugs are used to boost animal weight, but many experts believe the vast majority go toward non-medical uses.

The debate over antibiotics has long pitted the benefits for producing safe, low-cost meat against the risk of contributing to dangerous antibiotic-resistant bacteria that can infect humans.

The National Pork Producers Council said Wednesday that the FDA "did not provide compelling evidence" that antibiotic use in livestock is unsafe.